UMONITOR, UMWAIT and TPAUSE are a set of user wait instructions.

UMONITOR arms address monitoring hardware using an address. A store
to an address within the specified address range triggers the
monitoring hardware to wake up the processor waiting in umwait.

UMWAIT instructs the processor to enter an implementation-dependent
optimized state while monitoring a range of addresses. The optimized
state may be either a light-weight power/performance optimized state
(c0.1 state) or an improved power/performance optimized state
(c0.2 state).

TPAUSE instructs the processor to enter an implementation-dependent
optimized state c0.1 or c0.2 state and wake up when time-stamp counter
reaches specified timeout.

Availability of the user wait instructions is indicated by the presence
of the CPUID feature flag WAITPKG CPUID.0x07.0x0:ECX[5].

The patches enable the umonitor, umwait and tpause features in KVM.
Because umwait and tpause can put a (psysical) CPU into a power saving
state, by default we dont't expose it in kvm and provide a capability to
enable it. Use kvm capability to enable UMONITOR, UMWAIT and TPAUSE when
QEMU use "-overcommit cpu-pm=on, a VM can use UMONITOR, UMWAIT and TPAUSE
instructions. If the instruction causes a delay, the amount of time
delayed is called here the physical delay. The physical delay is first
computed by determining the virtual delay (the time to delay relative to
the VM’s timestamp counter). Otherwise, UMONITOR, UMWAIT and TPAUSE cause
an invalid-opcode exception(#UD).

The release document ref below link:\

        Remove CPUID_7_0_ECX_WAITPKG if enable_cpu_pm is not set.

Tao Xu (2):
  x86/cpu: Add support for UMONITOR/UMWAIT/TPAUSE
  target/i386: Add support for save/load IA32_UMWAIT_CONTROL MSR

 target/i386/cpu.c     |  2 +-
 target/i386/cpu.h     |  3 +++
 target/i386/kvm.c     | 19 +++++++++++++++++++
 target/i386/machine.c | 20 ++++++++++++++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)


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